Earlier quoted context omitted.
they become a problem for advertising networks when they are no-longer able to proved enough money to be worth the content creators time to accept/administrate. And when a network ad-spots start drying up, the advertisers will have to switch. at least, that's how I'd hope it would work.
Where do you expect the advertisers will switch to? Nah whats going to happen is that sites will just start doing one of two things. a) they will start blocking users that appear to be using ad blockers. b) they will start including ads in ways that make ad blockers ineffective. Embedding them in content or proxying the ads to come from the host itself. Ads aren't going away.
Many of us hope for option c - that publishers work with ad networks to fix some of the problems that drive people to ad blockers in the first place. Some people object to advertising no matter how it is done, but I think that's a relative small number of people. Most people will accept well-behaved ads.